The present invention is generally related to electrical induction machines and more specifically to a stator for an induction machine.
Conventionally, the stator assembly of an electrical machine has a stator core formed of a stack of steel laminations. As an alternative to the use of steel laminations, the stator core may be formed from iron powder, as exemplified by U.S. Pat. No. 4,947,065 disclosing a stator moulded in one-piece, and by International Patent Application WO95/12912 disclosing a stator comprising a plurality of separate and substantially identical parts.
By its very nature any compacted, non-sintered material will not be fully dense. This means that soft iron powder currently available will have a permeability that is lower than the permeability obtainable with steel laminations. However, magnetic powder composites could offer advantages such as isotropic magnetic behaviour, reduction in iron losses at high frequencies, improved thermal characteristics and flexible design and assembly.
The use of single tooth geometry could give rise to large benefits when it comes to thermal and manufacturing properties of electrical machines. However, the single tooth winding geometry will also give rise to a different spectrum of harmonics of the armature field, compared to a standard winding arrangement. These higher order fields, which may travel around the airgap at different speed compared to the working harmonics, will induce eddy currents in the stator and the rotor.
In a synchronous machine, these higher order fields have substantially no influence on the torque, while in an induction machine they will produce additional torque at synchronous speeds different to the main speed and thereby result in reductions and/or dips in the torque-speed characteristic and extra rotor losses.
In a traditional induction machine, this is avoided by distributing the windings in slots, but this is not possible if one wants to have a polyphase winding made from single tooth sections.
JP-A-7298578 discloses an alternative for the single tooth winding geometry. More specifically, the stator is divided into two parts along its axis and the two parts are shifted an angle of 0-120xc2x0 electrical, preferably 90xc2x0 electrical. However, this shift only cancels the second harmonics and therefore further measures are necessary. According to this reference, a fixed tooth width (or slot opening width) to tooth pitch ratio is necessary to cancel higher order even harmonics. This results in less geometrical freedom for the motor design. The conventional use of slot skew to reduce cogging also will be affected by the constricted motor design parameters.
One object of the present invention is to provide a stator for an electrical induction machine which benefits from the use of the single tooth geometry and at the same time corrects reductions and/or dips in the torque-speed characteristic and the extra rotor losses without resort to the features disclosed in JP-A-7298578.
This object is achieved by a stator as claimed in the appending claim 1. Thus, by dividing the stator into an even number of stator sections at different axial positions, each section having a plurality of circumferentially separated, radially oriented teeth and each tooth having a single winding, the effect of other harmonics than the working harmonics may be reduced in that the stator sections are mutually phase shifted by substantially 360xc2x0/n electricalxc2x1an angle related to skew and in that n/2 of the stator sections have their electrical supplies shifted by 180xc2x0 electrical.
The effect produced by several stator sections on a single rotor is substantially the same as a distributed winding. This leads to the cancelling of a large fraction of the higher harmonics while keeping the benefits of single tooth windings, i.e. high slot fill factor, and simple manufacturing and assembling.
The two stator sections will only be separated by a small air-gap of the order of the teeth opening, preferably obtained by making each stator section at least partly from a magnetic powder. The stator sections should at least have different axial positions; i.e. they might even be juxtaposed. Each stator section will contain the same harmonics, but the phase shifting of them will, seen from the rotor conductor bars, cancel a large fraction of unwanted higher order harmonics.